FIG. 1 illustrates a sectional view of a prior art heated capacitive pressure sensing device 100. Device 100 includes several major components such as an external metallic shell 110, an internal heater shell 120, a heater 130, a capacitive pressure sensor 140, an inlet tube 144, and electronics assemblies 170. In operation inlet tube 144 is connected to an external source of gas (not shown), and transducer 100 generates an output signal indicative of the pressure of the gas in inlet tube 144. The term “gas” is used herein to refer to any fluid.
For convenience of illustration, many mechanical details of transducer 100, such as the construction of sensor 140 and the mounting of sensor 140 and electronics assemblies 170, have been omitted from FIG. 1. However, heated capacitive pressure transducers such as transducer 100 are well known and are described for example in U.S. Pat. No. 5,625,152 (Pandorf); U.S. Pat. No. 5,911,162 (Denner); and U.S. Pat. No. 6,029,525 (Grudzien).
Pressure sensing device 100 is typically used in integrated circuit fabrication facilities to measure a pressure such as the pressure within a chemical vapor deposition chamber, or the pressure of a gas that is being supplied in controlled volumes to such a deposition chamber. The gasses in such facilities are typically carefully maintained at a particular temperature such as two hundred degrees Celsius (200° C.). Heaters are included in pressure sensing device 100 to heat the surfaces that contact the gas, such as the internal surfaces of sensor 140 and inlet tube 144, to the same temperature as the gas. This avoids condensation of the gas within pressure sensing device 100 and also reduces distortions in the pressure measurement provided by pressure sensing device 100.
External metallic shell 110 includes a lower sensor enclosure 112, an upper electronics enclosure 114, and a joiner 116 that holds enclosures 112, 114 together. Heater shell 120 is disposed within the lower enclosure 112 and includes a lower enclosure or can 122 and a cover 124. Pressure sensor 140 is disposed within heater shell 120. A temperature sensor (e.g., a thermistor) 190 is fixed to an internal surface of heater shell 120. Heater 130 is disposed on the external surface of heater shell 120 and includes a barrel heater 132 and an end heater 134. Barrel heater 132 is wrapped around the external cylindrical sidewall of can 122 and end heater 134 is disposed on the bottom of can 122. Barrel heater 132 and end heater 134 are electrically connected via wires 136 so the two heaters 132, 134 may be simultaneously controlled via a single electrical signal. Electronics assemblies 170 are disposed within the upper electronics enclosure 114.
Since the temperature of the gas 111 (e.g., 200° C.) is often too high for reliably operating electronics assemblies 170, the electronics assemblies 170 are located in an area that is remote from any of the surfaces within pressure sensing device 100 that will actually contact the gas. This allows the electronics assemblies 170 to operate at one temperature (e.g., 70° C.) while the sensor 140 is simultaneously operated at a higher temperature (e.g., 200° C.). Also, since human operators sometimes touch the external surfaces of transducer 100, to prevent injuries it is generally desirable to insure that the external surfaces remain below about 60° C. Thus, while it is in operation, transducer 100 may be characterized by three different operating temperatures. A first temperature (e.g., 200° C.), to which the surfaces that contact the gas are heated; a second temperature (e.g., 70° C.) at which the electronics assemblies 170 operate; and a third temperature (e.g., 60° C.) which the external shell 110 does not exceed.
The above noted U.S. Pat. No. 5,625,152 (Pandorf) discloses prior art transducers that use various combinations of heating, insulating, and ventilating, that enable the transducers to simultaneously operate at the three desired temperature ranges. Although those transducers have been generally successful, they are often bulkier than is desirable. Accordingly, there remains a need for improved thermal control in pressure sensing devices.